The prior art has many examples of pipe cutting or machining apparatus which can be mounted onto a pipe and includes a tool head carrying a selected tool for performing a particular desired cutting or machining operation, such as grooving or cutting-off. One such type of apparatus comprises a frame which can be easily and quickly mounted on the pipe, drive means for rotating the tool head around the pipe, and means for advancing the tool towards the pipe as the tool head is driven therearound.
One example of such an apparatus is the portable lathe shown in U.S. Pat. No. 4,543,861. Here a tool head, rotatable on a mandrel engageable within a pipe, carries a tool for either a cutting-off or bevelling operation. Advancement of the tool radially with respect to the pipe is achieved by a mechanism comprising a tool-advancing cam carried on a stationary element of the tool head and which intermittently transmits movement via a one-way clutch to the tool head through a rigid linkage including a feed screw associated with a tool slide. This tool-advancing mechanism is however relatively complex, because of the necessity for the linkage to be of variable length, so it can be adapted to cut the pipe at various angles.
Another example of a pipe cutting or machining apparatus is my own earlier design shown in published International patent application WO2007/052035. In this improved construction, which makes the job of cutting or machining a pipe quicker and easier for an operator, a two-part hinged holder clamps the pipe externally and a cutting head supported thereon is rotatable around the pipe to perform the cutting operation. The cutting head includes a blade whose advancement radially towards the pipe is governed by an indexing mechanism fixed to the frame and including a snail cam and pawl and ratchet device which incrementally advances the cutting blade radially inwardly towards the pipe's axis in response to completion of each revolution of the cutting head about the pipe.
Whilst the above apparatus of WO2007/052035 is an improvement in certain respects upon other known pipe cutting or machining apparatuses, it still relies on a relatively complex tool advancement mechanism, and one that demands quite high engineering of a relatively large number of interconnected moving parts. By its very nature therefore, the blade advancing mechanism is prone to high levels of wear through normal use, thus reducing its working life, and also propensity to damage or malfunction from ingress of dirt and foreign bodies that are frequently typical of environments in which the apparatus is to be used, eg. building sites.